The IRI has prepared this experimental Climate Outlook for North America for December 2001 - May 2002. Of relevance in the preparation of this outlook is the prediction of near-average conditions in the central and eastern equatorial Pacific through early March 2002. Currently the sea surface temperatures (SSTs) across much of the eastern and central equatorial are near their long-term average (SSTs), although slightly lower than average SSTs exist in the eastern portion of the tropical Pacific basin and warmer than average SSTs persist in the central and western parts of the basin. Thus, the Nino 4 region remains slightly above normal, Nino 3.4 is near normal, and Nino 3 and Nino 1+2 are slightly below normal. Overall, near neutral equatorial Pacific SST conditions are expected for the first two overlapping season s of the forecast, December-February 2002, January-March 2002, while during the two seasons February-April 2002, March-May 2002, they are expected to become near to slightly above average even in the eastern tropical Pacific with slightly below average SST becoming more limited to slightly south of the equator. The warmer than average SSTs that continue in much of the Indian Ocean are expected to decrease slowly toward normal through the forecast period. The area of above-average temperature in the tropical north Atlantic Ocean, and more weakly in part of the tropical south Atlantic, is expected to continue but weaken through the forecast period.

METHODS -

This Outlook was prepared using the following procedures and information:

A) Coupled ocean-atmosphere model predictions of tropical Pacific SST covering the forecast period. Particularly heavy weighting has been given to predictions from the coupled model operated by the NOAA National Centers for Environmental Prediction, Climate Modeling Branch. This model suggests a continuation of near-average conditions during the first two 3-month forecast seasons, turning toward slightly above normal SST in the latter two seasons. This forecast SST scenario is consistent with some, but not all, numerical and statistical forecasts of central and eastern Pacific SSTs.

B) Forecasts of the tropical Indian ocean using a statistical model developed by the IRI.

C) Global atmospheric general circulation model (GCM) predictions of the atmospheric response to the present and predicted sea-surface temperature patterns.

D) Other sources of information include CPC ; NASA's Seasonal to Interannual Prediction Project (GSFC-NASA) and also seasonal prediction research at COLA.

The procedures, models, and data used to derive this Climate Outlook may be somewhat different from those used by the national meteorological services in the region. Thus, this product may differ from the official forecasts issued in those areas. The Climate Outlook for July - December 2001 is dependent on the accuracy of the SST predictions. For the tropical Pacific, these predictions can be expected to provide useful information, but there is some uncertainty concerning the evolution of SSTs. Spread in global SST predictions is a source of uncertainty in the Outlook provided here. Note that even if perfectly accurate SST forecasts were possible, there would still be uncertainty in the climate forecast due to chaotic internal variability of the atmosphere. These uncertainties are reflected in the probabilities given in the forecast.

It is stressed that the current status of seasonal-to-interannual climate forecasting allows prediction of spatial and temporal averages, and does not fully account for all factors that influence regional and national climate variability. This Outlook is relevant only to seasonal time scales and relatively large areas; local variations should be expected, and variations within the 3-month season should also be expected. For further information concerning this and other guidance products, users are strongly advised to contact their National Meteorological Services.

OUTLOOK -

This Outlook covers two seasons: July - September 2001 and October - December 2001. Maps are given showing tercile probabilities of precipitation and temperature. The maps for precipitation indicate the probabilities that the seasonal precipitation will fall into the wettest third of the years (top number), the middle third of the years (middle number), or the driest third of the years (bottom number). The color shading indicates the probability of the most dominant tercile -- that is, the tercile having the highest forecast probability. The color bar alongside the map defines these dominant tercile probability levels. The upper side of the color bar shows the colors used for increasingly strong probabilities when the dominant tercile is the above-normal tercile, while the lower side shows likewise for the below-normal tercile. The gray color indicates an enhanced probability for the near-normal tercile (nearly always limited to 40%). As before, numbers and their associated histograms show the probabilities of the three terciles. In areas with lots of spatial detail, there may not be sufficient room on the map, to allow histograms for each region. In those cases, some idea of the probabilities may be gained from the color alone. A qualitative outlook of climatology ("C") indicates that there is no basis for favoring any particular category. Areas that are marked by "D" represent regions experiencing their dry season (less than 15% of annual precipitation) and for which less than 5cm of precipitation typically occurs over the season. Otherwise, for example, in the case of December-February 2002 (Map A), there is a 25% probability that the precipitation will be in the wettest third of the years, a 35% chance it will be in the near-normal third of the years, and a 40% chance that the precipitation will be in the driest third of the years in much of the southeastern U.S. and part of northern Cuba.

Maps of temperature show expected probabilities that the seasonal temperatures will fall into the warmest third of the years, the middle third of the years, or the coldest third of the years (Map A). The numbers for each region on the temperature maps indicate the probabilities of temperatures to fall in each of the three categories, above-, near-, and below-normal.

An additional precipitation map is provided for the first season indicating probabilities for extreme precipitation anomalies. Extremes are defined as anomalies that fall within the top and bottom 15th percentile of the observed records. A priori, there is a 15% probability of being within the extremely wet category, and a 15% probability of being within the extremely dry category, leaving a 70% probability that the precipitation will not be extreme. The maps indicate areas of increased risk of extreme precipitation totals. Three levels of increased risk are defined: slightly enhanced risk, enhanced risk, and greatly enhanced risk. For slightly enhanced risk, there is a 25-40% probability that precipitation will be within the indicated extreme, i.e. wet or dry. This represents an approximate doubling of the climatological risk. For enhanced risk, there is a 40-50% probability that precipitation will be within the indicated extreme. This represents an approximate tripling of the climatological risk. For greatly enhanced risk, the probability that precipitation will be within the indicated extreme exceeds 50%, i.e. the indicated extreme is the most likely outcome. A similar map is provided in the first season indicating probabilities of extreme temperature anomalies.

Boundaries between sub-regions should be considered as transition zones, and their location considered to be only qualitatively correct.

December-February 2002 through March-May 2002:

The following discussion briefly describes the probability anomaly forecasts:

Slightly enhanced probabilities for above normal precipitation are forecast for various parts of mainly southern Alaska and neighboring northwestern Canada through the February-March-April period. The same outlook is being issued for extreme northwestern Mexico/southwestern U.S. for the December- January-February period only. A slightly enhanced probability for below normal precipitation is forecast for the southern and southeastern U.S. for December-January-February through February-March-April, being most pronounced in January-February-March when probabilities for below normal rise to 45% over the extreme southeastern U.S. and part of the Bahama Islands. In February-March-April the pattern becomes weaker and more scattered, and includes parts of northwestern Mexico that are forecast to have a slightly enhanced probability for above normal precipitation in December-January-February. A greatly enhanced probability for below normal precipitation (50%) is forecast for southern Latin America for December-January-February, surrounded by more weakly enhanced probabilities for below normal to the north in the middle part of Latin America. This pattern continues more weakly into the January-February-March period. Slightly enhanced probabilities for below normal precipitation are forecast for the Windward Islands of the Caribbean for all four forecast periods.

A greatly enhanced probability for above normal temperature (50%) is forecast for parts of the Caribbean islands throughout most of the forecast periods, centered on the Dominican Republic and Haiti, and immediate surroundings. This forecast probability anomaly is caused mainly by the above normal SST presently in the northern tropical Atlantic, which is expected to dissipate only slowly over the next several months. More weakly enhanced probabilities of above normal temperature are forecast for various other portions of North America throughout the four forecast periods, including parts of the western U.S., parts of Alaska, southwestern Greenland, and parts of mainly eastern Canada. Probabilities for above normal temperature rise to 50% in eastern Newfoundland in December-January-February, caused by much above normal SST in the adjacent Atlantic waters that is expected to slowly return toward normal in the coming several months.

OBSERVED CLIMATOLOGY DATA for Dec-Jan-Feb, Jan-Feb-Mar, Feb-Mar-Apr and Mar-Apr-May

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